The present invention relates generally to permanent magnet generators and control systems, and more particularly to protection schemes for permanent magnet generators.
In conventional systems that utilize a synchronous wound field generator, output voltage generated by the synchronous wound field generator is a function of the speed of the rotor and the excitation provided to the wound field. The output voltage of the wound field generator is controlled by selectively increasing or decreasing the excitation provided to the wound field. Likewise, protection of the wound field generator during fault conditions is simply a matter of removing the excitation provided to the wound field.
However, synchronous wound field generators do not offer the efficiency or robustness of other types of generators, such as permanent magnet generators. Advances in magnetic materials have made permanent magnet (PM) generators (in which the wound field is replaced with permanent magnets) increasingly popular. The permanent magnets provide the excitation flux needed to generate electricity in the armature windings. The absence of the rotating windings and diode assemblies required in the synchronous wound field generators make the PM generator inherently more rugged, efficient, and reliable. However, because the excitation flux generated by the permanent magnets is constant, the output voltage generated by a PM generator is a function only of the load and operating speed of the machine driving the rotor. Unlike synchronous wound field generators in which protection of the generator during a fault condition is a matter of simply removing the excitation provided to the wound field, the permanent magnets of the PM generator provide constant excitation. Therefore, in order to protect a PM generator, the rotation of the rotor must be reduced or stopped. The problem is further complicated if the PM generator receives mechanical power from a hydraulic motor. For instance, if the motor is a hydraulic motor, then a valve must be closed in order to reduce the mechanical power being provided to the PM generator. Therefore, protection schemes must be capable of quickly detecting and responding to fault conditions.